Hydrogen Sulfide (H2S) is a toxic gas that places restrictions on the materials than can be used for piping and other equipment handling sour gas, as many metals are sensitive to sulfide stress cracking. The presence of H2S in gas causes lower quality burning and the production of sulfur dioxide, and so is regulated in commercially sold gas.
H2S has an offensive odor of “rotten eggs” at concentrations as low as 50 parts per billion by volume (ppbv) and is toxic at concentrations above 1000 parts per million by volume (ppmv). H2S is a health and safety hazard, and when combined with carbon dioxide (CO2) and water vapor (H2O), corrodes plant equipment such as boilers and piping, and can ruin power-generating equipment. Energy production is hampered if H2S gas is present. This is especially true as the cost of energy increases.
Sour gas is natural gas or any other gas containing significant amounts of H2S. Natural gas is usually considered sour if there are more than 5.7 milligrams of H2S per cubic meter of natural gas, which is equivalent to approximately 4 ppm by volume.
Although the terms acid gas and sour gas are used interchangeably, strictly speaking, a sour gas is any gas that contains H2S in significant amounts, whereas an acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide (CO2) or H2S. Thus, carbon dioxide by itself is an acid gas but it is not a sour gas.
Before a raw natural gas containing H2S and/or carbon dioxide can be used, the raw gas must be treated to remove those impurities to acceptable levels, commonly by an amine gas treating process. The removed H2S is most often subsequently converted to by-product elemental sulfur in a Claus process.
Processes within oil refineries or natural gas processing plants that remove mercaptans and/or H2S are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and H2S.